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Water Cannons from WikiPedia:

A water cannon is a device that shoots a high-pressure stream of water. Typically, water cannons can deliver a large volume of water, often over dozens of metres / hundreds of feet. They are used in firefighting and riot control. Most water cannons fall under the category of a fire monitor.

Water cannon were first devised for use on fireboats. Extinguishing fires on boats and buildings near the water was much more difficult and dangerous before fireboats were invented. The first fireboat deployed in Los Angeles was commissioned on August 1, 1919. The first fireboat in New York City was Marine 1, deployed February 1, 1891. There may have been other fireboats elsewhere even earlier.

Fire trucks deliver water with much the same force and volume of force as a water cannon, and have even been used in riot control situations, but are rarely referred to as water cannon outside this context.

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